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A colorimetric, label-free, and nonaggregation-based gold nanoparticles (Au NPs) probe has been developed for the detection of [Pb.sup.2+] in aqueous solution, based on the fact that [Pb.sup.2+] ions accelerate the leaching rate of An NPs by thiosulfate ([S.sub.2][O.sub.3.sup.2-]) and 2-mereaptoethanol (2-ME). Au NPs reacted with [S.sub.2][O.sub.3.sup.2-] ions in solution to form Au[([S.sub.2][O.sub.3]).sub.2.sup.3-] complexes on the All NP surfaces, leading to slight decreases in their surface plasmon resonance (SPR) absorption. Surface-assisted laser desorption/ ionization time-of-flight ionization mass spectrometry (SALDI-TOF MS) data reveals the formation of Pb--Au alloys on the surfaces of the Au NPs in the presence of [Pb.sup.2+] ions and 2-ME. The formation of Pb--Au alloys accelerated the Au NPs rapidly dissolved into solution, leading to dramatic decreases in the SPR absorption. The 2-ME/[S.sub.2][O.sub.3.sup.2-]-Au NP probe is highly sensitive (LOD = 0.5 nM) and selective (by at least 1000-fold over other metal ions) toward [Pb.sup.2+] ions, with a linear detection range (2.5 nM-10 [micro]M) over nearly 4 orders of magnitude. The cost-effective probe allows rapid and simple determination of the concentrations of [Pb.sup.2+] ions in environmental samples (Montana soil and river), with results showing its great practicality for the detection of lead in real samples. 10.1021/ac9018268